Wafer notch detection

ABSTRACT

Notch detection methods and modules are provided for efficiently estimating a position of a wafer notch. Capturing an image of specified region(s) of the wafer, a principle angle is identified in a transformation, converted into polar coordinates, of the captured image. Then the wafer axes are recovered from the identified principle angle as the dominant orientations of geometric primitives in the captured region. The captured region may be selected to include the center of the wafer and/or certain patterns that enhance the identification and recovering of the axes. Multiple images and/or regions may be used to optimize image quality and detection efficiency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is filed under 35 U.S.C. § 120 and § 365(c) as acontinuation of International Patent Application Serial No.PCT/US15/15270, filed on Feb. 10, 2015, which application claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional Patent ApplicationNo. 61/939,131 filed on Feb. 12, 2014, which applications areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor technology,and more particularly, to identification of the position of the wafernotch.

BACKGROUND OF THE INVENTION

Typically, the wafer orientation is conveyed by the position of thenotch, which indicates the crystallographic orientation of wafer. In theprior art, wafer orientation computation based on notch orientation istime consuming, as it requires exhaustive search in the full angularrange of 360 degrees. Typically, the result of wafer orientation basedon notch alone is limited in accuracy and requires additional step of“Fine Alignment,” which is time consuming. In order to avoid longmechanical movements, some of the existing solutions require additionalhardware such as extra sensors, or an additional camera which covers alarge part of the wafer within its field of view or several cameras. Allthose exhibit added complexity and cost.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention provides a method of estimating aposition of a wafer notch, the method comprising capturing an image ofone or more specified region(s) of the wafer, identifying a principleangle in a transformation of the captured image which is converted intopolar coordinates, and recovering one or more wafer axis (or axes) froman identified principle angle.

These, additional, and/or other aspects and/or advantages of the presentinvention are set forth in the detailed description which follows;possibly inferable from the detailed description; and/or learnable bypractice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the invention and to showhow the same may be carried into effect, reference will now be made,purely by way of example, to the accompanying drawings in which likenumerals designate corresponding elements or sections throughout.

In the accompanying drawings:

FIG. 1 is a high level schematic illustration of a wafer with its notchand a notch detection module associated with an optical system within ametrology system, according to some embodiments of the invention;

FIG. 2 is a high level schematic illustration of intermediate steps in astreet orientation algorithm, operable by the notch detection module,according to some embodiments of the invention;

FIG. 3 is an illustration of exemplary inputs onto which the streetorientation algorithm may be applied, displaying increasing levels ofnoise, according to some embodiments of the invention;

FIG. 4 is an illustration of exemplary input images from different waferregions for selection according to their characteristics, according tosome embodiments of the invention; and

FIG. 5 is a high level schematic flowchart of a method, according tosome embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Prior to the detailed description being set forth, it may be helpful toset forth definitions of certain terms that will be used hereinafter.

The term “geometric primitives” in an image, as used in this applicationrefers to basic forms, objects and patterns in the image, such as lines,simple shapes or recurring elements.

The term “street orientation” as used in this application refers toorientations of geometric primitives in the image, e.g., with respect toa given grid. The term “street orientation algorithm” as used in thisapplication refers to ways of deriving the street orientation.

It is further noted, that there is a strong geometrical correlationbetween one wafer axis and the other wafer axes, and respectivelybetween the principle angle to one wafer axis and the principle anglesto the other wafer axes. Geometrically, the wafer axes are separated bymultiples of 90° as are the principle angles with respect to the waferaxes. Hence, in the following description, any aspect relating to onewafer axis and/or one principle angle is to be understood as potentiallyrelating to any number of wafer axes and/or principle angles. Forexample, any orientation measurement may be carried out with respect toone or more wafer axes.

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

Before at least one embodiment of the invention is explained in detail,it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is applicable to other embodiments or of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting.

Notch detection methods and modules are provided for efficientlyestimating a position of a wafer notch. Capturing an image of specifiedregion(s) of the wafer, a principle angle is identified in atransformation, converted into polar coordinates, of the captured image.Then the wafer axes are recovered from the identified principle angle asthe dominant orientations of geometric primitives in the capturedregion. The captured region may be selected to include the center of thewafer and/or certain patterns that enhance the identification andrecovering of the axes. Multiple images and/or regions may be used tooptimize image quality and detection efficiency. The angular orientationof the wafer is measured with high accuracy in minimal time, bysimultaneous minimization of algorithmic complexity and mechanicalmovements of the vision system with respect to the wafer. The notchposition may be detected implicitly, without directly sensing it.Certain embodiments overcome the challenges produced by the toolarchitecture such as: limited field of view of the vision system and thetime-consuming mechanical movements for positioning of the camera withrespect to the wafer both lateral and rotational, as well as imagecorruption by various types of noise.

It is assumed that the orientation of the wafer at the beginning of theprocedure is arbitrary and that the wafer deviation from the chuckcenter is limited to a 2 mm error. The computation of wafer orientationmay be triggered in the system as part of either “Train” or “Run”sequences. The time optimization of the “Run” sequence is the mostcritical because it directly affects the throughput of the tool.Disclosed modules and methods use a Street Orientation Algorithm toreduce the search space of orientation and notch position to fourpoints, for example, a robust algorithm for Street Orientation may beused to enable skipping the fine alignment step. In addition, anchorpoints may be used to avoid long mechanic movements from center ofwafer, as described below.

FIG. 1 is a high level schematic illustration of a wafer 60 with itsnotch 70 and a notch detection module 107 associated with an opticalsystem 105 within a metrology system 100, according to some embodimentsof the invention. It is noted that disclosed methods and modules may beapplied to systems in the semiconductor industry other than metrologysystems and that the latter merely serve in the present disclosure as anon-limiting example.

The orientation of wafer 60 is uniquely defined by the position of itsnotch 70. The periodic layout of devices printed on wafer 60 and theborders of dies are aligned along the Cartesian axes 61, 62, with notch70 is at the end-point of y axis 62.

Instead of detecting notch 70 visually at the periphery of wafer 60,e.g., by imaging whole wafer 60 with respective axes 61, 62 and center65, or imaging the wafer periphery, notch detection module 107 merelyimages 110 a central region 115 of wafer 60, which may include wafercenter 65 or not, and derives from the imaged region the orientations111, 112 of wafer axes 61, 62. For example, central region 115 maycomprise unique pattern(s) with known position with respect to center ofwafer 65. Notch detection module 107 uses derived orientations 111, 112to suggest possible locations 71A-71D for notch 70, and in certainembodiments also proceeds to determine which of locations 71A-71D is theactual notch location. Image(s) 110 of central region(s) 115 may beselected from multiple image(s) 110 and/or multiple regions 115, e.g.,according to image quality, derivation quality, or derivations frommultiple image(s) 110 and/or multiple regions 115 may be statisticallyanalyzed to derive more accurate location estimations.

Notch detection module 107 may implement any of the following componentsof the orientation computation task:

i. A “street orientation algorithm” may be used to find the dominantorientation (designated by the angle θ) of geometric primitives in thefield of view (FOV) of imaging device 100, for example with respect toimage 110 acquired at an estimated center of wafer 60.ii. A “notch detection algorithm” may comprise searching specified notchpattern(s) in up to three (out of four) possible locations 71A-71D atthe edge of wafer 60 according to detected orientations 111, 112.iii. An “anchor point training algorithm” may comprise searching anddefining the unique pattern(s) for selection of region 115, e.g., in thenear proximity to center 65 of wafer 60.iv. An “anchor point detection algorithm” may comprise pattern searchingof anchor point template(s) near center 65 of wafer 60 as basis for theanchor point training algorithm.

It is noted that the estimation of street orientation and hence of theorientation of the wafer axes may be carried out by itself.Additionally, estimation of the notch location and/or usage of anchorpoints or patterns as proxies for the notch location may be applied toderive the notch location from the street orientation without applyingdirect imaging of the notch region.

FIG. 2 is a high level schematic illustration of intermediate steps in astreet orientation algorithm 101, operable by notch detection module107, according to some embodiments of the invention. In certainembodiments, street orientation algorithm 101 relies on image analysisin the Fourier domain. Unlike other edge-based approaches the inventorsfound this method to be extremely fast and robust to noisy, out-of-focusand low-contrast inputs (images 110). The steps of proposed algorithm101 are illustrated in FIG. 2.

Upon captured image 122, shown in schematic wafer coordinates anddesignated 1(x,y), the 2D (two dimensional) Discrete Fourier Transformis applied and the absolute value of the Fourier coefficients arecomputed and presented as an image 124, denoted J(w,u). Then, J(w,u) isconverted to polar coordinates to yield J_(P)(r,θ) (illustrated in image126) and the orthogonal projection thereof onto the θ axis 128 is usedfor the orientation recovery using the resulting peaks separated by 90°,giving θ and θ+90°. Deriving θ, rotated image 130 may be generated fromcaptured image 122, with rotated image 130 characterized by orientations111, 112 that are congruent to wafer axes 61, 62. Notch 70 is located atone of the ends of one of orientations 111, 112. In certain embodiments,an ambiguity in the relative angle between orientations 111, 112 andwafer axes 61, 62 (among θ 71A, θ+90° 71C, θ+180° 71D and θ+270° 71B)may be resolved via notch pattern search (in “Train” sequences) oranchor pattern search (in “Run” sequences), both possibly implemented asmulti-scale rigid template matching with known scale and orientation.

The inventors have performed an accuracy and time requirements analysis,exemplified in the following by a few experimental results whichdemonstrate the robustness of proposed street orientation approach 101to eliminate the need for any additional fine alignment step.

FIG. 3 is an illustration of exemplary inputs 150 onto which streetorientation algorithm 101 may be applied, displaying increasing levelsof noise, according to some embodiments of the invention. Streetorientation algorithm 101 is applied on input image 122 withsynthetically added noise of varying level (single run) to yield images122A-122F with increasing levels of noise (the black and whiteillustrations miss some of the color coded information, especially inimages 122C, 122D). The mean and the standard deviation of theintensities of input images 122 are standardized to be between 0 and 1with noise standard deviation ranging from 0 (122A, resulting inaccurate θ=−40.481° for the specifically illustrated example) through 1(122B, θ=−40.486°), 2 (122C, θ=−40.471°) and 4 to 6 (122D-F andθ=−40.486°, θ=−40.504° and θ=−45.000° respectively). The inventors thusobserved that even under very heavy added white noise with standarddeviation 5 (122E), the error in θ resulting from street orientationalgorithm 101 and/or notch detection module 107 is much smaller than0.25°, which practically eliminates the need of fine alignment in system100 under many circumstances.

FIG. 4 is an illustration of exemplary input images 155 from differentwafer regions 115, for selection of image 122 according to theircharacteristics, according to some embodiments of the invention. It isnoted that different wafer regions 115 may comprise different regions onone wafer 60 or on different wafers 60.

In certain embodiments, multiple images 122G-J may be captured fromdifferent regions 115 on wafer and one or more images 122 may beselected therefrom by notch detection module 107 for applying the notchdetection analysis such as street orientation algorithm 101. Inillustrated non-limiting example 155, input images 122G-J may beacquired on wafer 60 with arbitrary orientations at a specified numberof different locations. Since no additional rotation of wafer 60 isapplied between acquisitions the orientation result (θ and/or notchposition) is expected to be the same so that the results from multipleimages 122 may be compared and analyzed, e.g., statistically bymeasuring scattering metrics of the results. Experiments may be repeatedfor several wafers 60, at various orientations, various imagingconditions (contrast and focus) to optimize the selection of region(s)115. In addition, comparing edge-based algorithm (EB) to FourierTransform-based algorithms (FTB) 101 illustrates the superiority of thelatter.

In 29 images 122 taken from different wafers 60 and/or different regions115, and with respect to accuracy metrics of range (of measured anglesθ)>0.3° and standard deviation (of measured angles θ)>0.2°, all FTBmeasurements conformed with both metrics, while in 11 and 9 EBmeasurements the metric's threshold were exceeded (respectively). FIG. 4illustrates as examples image 122G suffering from insufficientillumination and low contrast (in FTB θ_(range)=0.050 θ_(STD)=0.018while in EB θ_(range)=0.200 θ_(STD)=0.057), image 122H suffering fromsaturation and low contrast (in FTB θ_(range)=0.112 θ_(STD)=0.032 whilein EB θ_(range)=0.400 θ_(STD)=0.119), image 122I being out of focus (inFTB θ_(range)=0.031 θ_(STD)=0.032 while in EB θ_(range)=0.780θ_(STD)=0.266) and image 122J exhibiting atypical wafer design (in FTBθ_(range)=0.105 θ_(STD)=0.032 while in EB θ_(range)=0.390θ_(STD)=0.168), all exhibiting the better performance of the disclosedinvention. It is noted that the results of street orientation algorithm101 were stable even for out-of-focus and low-contrast inputs 122 andway below the limits which require additional refinement steps. It isfurther noted that while from a theoretical point of view, the mosttime-consuming operator is the Fourier Transform (O(N log N)), where Nis the number of pixels in input image 122, yet in practice, thecomputation time is negligible with comparison to the time required forthe mechanical movement of the camera in optical system 100 with respectto wafer 60.

FIG. 5 is a high level schematic flowchart of a method 200, according tosome embodiments of the invention. Method 200 comprises estimating aposition of a wafer notch (stage 210) and may be at least partiallycarried out by at least one computer processor 99 (stage 280).

Method 200 may comprise capturing an image of a specified region of thewafer (stage 220), e.g., capturing an image of a central region of thewafer (stage 222), and possibly capturing multiple images and selectingimages for further processing according to image characteristics (stage225). Method 200 may employ any algorithm for finding the dominantorientation (designated by the angle θ) of geometric primitives in thefield of view (FOV) of imaging device 100, for example with respect toimage 110 acquired at an estimated center of wafer 60 (stage 228).

Method 200 may further comprise transforming the captured image (stage230), calculating Fourier transform coefficients for the captured image(stage 235), converting the transformed image into polar coordinates(stage 240) and projecting the converted transformed image orthogonally(stage 245), and may further comprise identifying principle angle(s) inthe transformation of the captured image which is converted into polarcoordinates (stage 250).

In certain embodiments, method 200 comprises recovering wafer axis(es)from the identified principle angle(s) (stage 260) and identifying thewafer notch from the recovered wafer axis(es) (stage 270), for exampleby searching specified notch pattern(s) along the recovered waferaxis(es) (stage 272), by searching and defining unique pattern(s) to becaptured in the image, which allow notch identification (stage 274) andrespective selection of the captured region, e.g., in the near proximityto the center of the wafer and/or by pattern searching of anchor pointtemplate(s) that indicate the axis along which the notch is located(stage 276).

Advantageously using street orientation 101 to calculate waferorientation narrows down the search space of orientations to fourpossibilities and the quality of street orientation 101 exemplifiedabove eliminates the need for extra time required for a fine alignmentstep. Consecutively, using anchor points allows performing shortmechanical movement to identify the quadrate, i.e., and respectively theaxis along which the notch is located. For example, an anchor pointclose to the center of wafer may be selected to allow little or notravelling of the optical head, which is advantageous with respect toprior art movement to the location of the notch at the edge of wafer.Method 200 thus requires short stroke movements and hence shorteroperation time.

In the above description, an embodiment is an example or implementationof the invention. The various appearances of “one embodiment”, “anembodiment”, “certain embodiments” or “some embodiments” do notnecessarily all refer to the same embodiments.

Although various features of the invention may be described in thecontext of a single embodiment, the features may also be providedseparately or in any suitable combination. Conversely, although theinvention may be described herein in the context of separate embodimentsfor clarity, the invention may also be implemented in a singleembodiment.

Certain embodiments of the invention may include features from differentembodiments disclosed above, and certain embodiments may incorporateelements from other embodiments disclosed above. The disclosure ofelements of the invention in the context of a specific embodiment is notto be taken as limiting their used in the specific embodiment alone.

Furthermore, it is to be understood that the invention can be carriedout or practiced in various ways and that the invention can beimplemented in certain embodiments other than the ones outlined in thedescription above.

The invention is not limited to those diagrams or to the correspondingdescriptions. For example, flow need not move through each illustratedbox or state, or in exactly the same order as illustrated and described.

Meanings of technical and scientific terms used herein are to becommonly understood as by one of ordinary skill in the art to which theinvention belongs, unless otherwise defined.

While the invention has been described with respect to a limited numberof embodiments, these should not be construed as limitations on thescope of the invention, but rather as exemplifications of some of thepreferred embodiments. Other possible variations, modifications, andapplications are also within the scope of the invention. Accordingly,the scope of the invention should not be limited by what has thus farbeen described, but by the appended claims and their legal equivalents.

What is claimed is:
 1. A method of wafer notch detection comprising:capturing an image, with an imaging device, of at least one specifiedregion of the wafer; performing, with one or more processors, a streetorientation procedure comprising: performing a transformation of theimage; converting the transformation of the image into polarcoordinates; and determining a plurality of candidate locations of thenotch based on an orientation of geometric primitives in thefield-of-view of the imaging device; and identifying, with one or moreprocessors, a location of the notch by performing one or more notchpattern searches on at least a portion of the plurality candidatelocations.
 2. The method of claim 1, wherein the determining a pluralityof candidate locations of the notch based on an orientation of geometricprimitives in the field-of-view of the imaging device comprises:determining a plurality of candidate locations of the notch based on adominant orientation of geometric primitives in the field-of-view of theimaging device.
 3. The method of claim 1, wherein the specified regionof the wafer includes a center of the wafer.
 4. The method of claim 1,wherein the transformation is a two dimensional Discrete FourierTransform.
 5. The method of claim 1, further comprising: performing,with one or more processors, an anchor point detection procedure.
 6. Themethod of claim 1, wherein the identifying, with one or more processors,a location of the notch by performing one or more notch pattern searcheson at least a portion of the plurality candidate locations comprisessearching at least one specified notch pattern along at least onederived wafer orientation.
 7. The method of claim 1, further comprising:performing, with one or more processors, an anchor point trainingprocedure, wherein the anchor point training procedure includessearching and defining at least one unique pattern to be captured in theimage, to facilitate notch identification.
 8. The method of claim 1,further comprising: capturing a plurality of images; and selectingtherefrom images for further processing according to imagecharacteristics.
 9. A system for wafer notch detection comprising: anoptical system, wherein the optical system is configured to capture animage of at least one selected region of a wafer, and a computerprocessor configured to: receive, from the optical system, the image ofat least one specified region of a wafer; perform a street orientationprocedure comprising: performing a transformation of the image;converting the transformation of the image into polar coordinates; anddetermining a plurality of candidate locations of the notch based on anorientation of geometric primitives in the field-of-view of the imagingdevice, wherein the computer processor is further configured to:identify a location of the notch by performing one or more notch patternsearches on at least a portion of the plurality candidate locations. 10.The system for notch detection of claim 9, wherein the transformation isa two dimensional Discrete Fourier Transform.
 11. The system for notchdetection of claim 9, wherein the computer processor is furtherconfigured to performing an anchor point detection procedure.
 12. Thesystem for notch detection of claim 11, wherein the computer processoris further configured to identify a location of the notch by performingone or more notch pattern searches on at least a portion of theplurality candidate locations by searching at least one specified notchpattern along at least one derived wafer orientation.
 13. The system fornotch detection of claim 9, wherein the computer process is furtherconfigured to perform an anchor point training procedure, wherein theanchor point training procedure includes searching and defining at leastone unique pattern to be captured in the image, to facilitate notchidentification.
 14. The system for notch detection of claim 9, whereinthe optical system is configured to capture a plurality of images,wherein the computer processor is configured to select from theplurality of images a set of images for further processing according toone or more image characteristics.
 15. The method of claim 3, whereinthe center of the wafer contains one or more patterned structures,wherein the patterned structures are aligned with one or more axes ofthe wafer.
 16. The method of claim 8, wherein the plurality of imagesare captured from different portions of the wafer.
 17. The method ofclaim 1, wherein the imaging device comprises a camera within ametrology system.
 18. The system of claim 9, wherein the center of thewafer contains one or more patterned structures, wherein the patternedstructures are aligned with one or more axes of the wafer.
 19. Thesystem of claim 14, wherein the plurality of images are captured fromdifferent portions of the wafer.
 20. The system of claim 9, wherein theoptical system comprises an optical system within a metrology system.